I. Field of the Invention
This invention relates to an improved process for phosphorylating starch. More particularly, this invention is directed to a continuous process for preparing orthophosphate starch monoesters using a rotary vacuum filter.
II. Description of the Prior Art
Phosphorylation reactions of starch, wherein starch is impregnated with a phosphate salt and thereafter dried and heat-reacted to obtain an orthophosphate starch monoester, are well known in the art.
Numerous patents such as U.S. Pat. Nos. 2,806,026; 2,824,870; 2,884,412; 2,884,413; 2,961,440; and 3,132,066 disclose various phosphorylation techniques whereby the impregnation step is accomplished by adding the phosphate salt to the starch, either by spraying an aqueous solution of the salt on the dry starch or by adding salt to an aqueous slurry of starch and filtering or centrifuging the starch slurry, then drying by conventional means. Spray-drying may be used in place of filtration or centrifugation but it is a very costly operation.
The suspension method of impregnation, which was deemed necessary to insure even distribution and/or penetration of the reagent into the starch granules, has serious disadvantages because of the resulting filtrates which must be disposed of. The excess phosphate salt which does not penetrate the starch is lost to the filtrate and fed into the effluent, creating serious pollution problems. In commercial processes about 60% of the phosphate salt in the reagent solution is lost in the effluent.
In an effort to avoid the pollution problems caused by suspension impregnations, an alternative method of impregnation of starch with phosphate salt was proposed in E. Ger. Pat. No. 36,806. This method involves spraying dry, powdered starch with a concentrated solution of the phosphate salt or blending a wet starch cake with dry, powdered phosphate salt. In a third variation of this pollution-free impregnation, dry starch and dry salt are blended together and then sprayed with water to achieve dissolution of the phosphate salt and to distribute it evenly throughout the starch.
While the E. Ger. process represents an improvement over the suspension method of impregnation because pollution is minimized, it has several inherent disadvantages. Thus, the variation wherein dry reagent is added to wet starch cake requires special delivery equipment to sift the powdered reagent onto the wet starch. Blending dry starch with dry reagent requires prolonged mixing to achieve uniform distribution of the phosphate salt throughout the starch powder, which can result in poor granule penetration and, thus, low reaction efficiency. Moreover, the variation of spraying dry starch with phosphate salt solution, in addition to having the above disadvantage, involves a double drying procedure wherein the starch must be dried before use and dried again after spraying, adding appreciably to the cost of the process.
A further disadvantage of the E. Ger. process is that all of the mixing takes place in a blender so that metering of the reagent must be done carefully or the mixing done over a prolonged period of time to insure satisfactory distribution of the reagent within the starch. This becomes more critical when lower levels of substitution are desired and smaller amounts of reagent thus employed. Also such a blender is not a part of the processing equipment so that the phosphorylation process must be interrupted using this procedure.
Alkali metal tripolyphosphate salts are the preferred phosphate salts for phosphorylation reactions because they require a lower heat input for satisfactory phosphorylation and are, thus, most desirable from an energy standpoint. However, sodium tripolyphosphate, which is representative of this type of salt, has an overall solubility in water at 25.degree. C. of only about 13% (see John von Wazer, "Phosphorus and Its Compounds", Vol. I: Chemistry, New York: Interscience Publishers, Inc., 1958, pp. 649-650). Thus, conventional sodium tripolyphosphate solutions of the prior art are relatively dilute.
U.S. Pat. No. 4,166,173 to O. B. Wurzburg et al. discloses an improved pollution-free process for phosphorylating starch using a concentrated reagent solution of alkali metal tripolyphosphate salt. The concentrated solution is used directly to impregnate a starch cake, with the impregnation step preferably carried out in a centrifuge in a semi-continuous process.